The interlaced scanning system is used widely for the conventional television systems. In the interlaced scanning system, a one frame image is composed of two fields; an odd field and an even field. Therefore, in an NTSC system television signal, for example, the number of scanning lines per frame image is 525 lines, and in each field, the number is halved to 262.5 lines. In a displayed image obtained with the interlaced scanning system, vertical scanning lines are generally rough and conspicuous, thus causing deterioration of the quality of displayed images.
Progressive scanning television receivers also have been proposed in the art. In a progressive scanning television receiver the horizontal line rate is doubled, and each line of frame image is displayed twice. As a result, a displayed image has twice the usual number of scanning lines found in the interlaced scanning system. Thus, various adverse influences on the displayed image, such as the visibility of the vertical line structure and line flicker caused by the interlaced scanning system, are reduced. Accordingly, progressive scanning television receivers facilitate the overall improvement of picture quality.
In one known form of progressive scanning television receiver, a scanning conversion circuit, as shown in FIG. 1, is provided. In the scanning conversion circuit each horizontal line of video signal is stored in one of two line memories 10a and 10b. The video input signal is alternatively applied to the line memories 10a and 10b through a video input switch 12 and read from the line memories 10a, 10b through a video output switch 14. As a line signal of the video input signal is stored in one memory 10a the other line signal of the video signal previously stored in the other memory 10b is read twice. Switch positions are then reversed with the video input signal being stored in the line memory 10b and read twice from the line memory 10a. Clock signals for controlling read/write operations of the line memories 10a, 10b are provided by a clock source 16, which provides a write clock signal to a multiplier 18. The multiplier 18 doubles the frequency of the write clock signal to produce a read clock signal. That is, the frequency of the read clock signal is two times of the frequency of the write clock signal. The write and read clock signals are applied to respective ones of the line memories 10a, 10b by means of a clock signal switch 20 synchronized with the video input switch 12 and the video output switch 14, such that the memory which is reading is clocked at twice the rate of the memory which is writing, thereby doubling the line rate of the video output signal.
As one horizontal line of the video signal is being stored in one line memory 10a, the horizontal line previously stored in the other memory 10b is recovered or read twice, thereby providing two lines of time-compressed video signal within one standard line interval in the interlaced scan line rate system. The memory output is applied to a display having a doubled horizontal scan rate synchronized with the readout of the memory 10a, 10b, thereby doubling the number of displayed lines of the video signal, compared to the interlaced scan line rate system.
In the NTSC system television signal, odd and even field video signals having each 262.5 scanning lines are transmitted by interlaced scanning per 1/60 sec., thereby to form a one frame image of 525 scanning lines from two of odd and even field video signals. In other words, video signals of two different fields are composed in each 1/60 sec., thereby to form a frame image in the period of 1/30 sec. A scanning converter is adapted to store the transmitted field video signals at prescribed times and read the same to convert the scanning of two field video signals forming one frame, from interlaced scanning to progressive scanning. This reproduces a high quality picture identical in field frequency to the interlaced scanning system and doubled in the number of scanning lines. In progressive scanning, one of the doubled scanning lines appears between normal upper and lower scanning lines within the same field.
The progressive scanning conversion system, however, has a problem in that a change of picture tones of the displayed images compared to the original image may occur under certain conditions. Such a problem in progressive scanning conversion systems is described in an article entitled, "Interlaced scanning conversion in a field deteriorates quality of displayed image" in "Nikkei Electronics", Oct. 20, 1986, pages 190-191.
The problem in progressive scanning conversion systems will be briefly described with reference to an extreme example. If frame images have a striped pattern in which white stripes and gray stripes correspond to scanning lines in the odd field and the even field, respectively, displayed images reproduced in response to the odd fields appear as plain white tone images over the entire display. On the other hand, displayed images reproduced in response to the even fields appear as plain gray tone images over the entire display.
The displayed images of both the plain white tone image and the plain gray tone image are viewed as overlapped by the human eye. The displayed images are viewed as plain light gray tone images over the entire display. Thus, the original image with such a striped pattern is changed to a different image by the progressive scanning system. This is because human eyes have a filter characteristic.